Tool for producing a crown wheel which can mesh with a pinion with oblique teeth, and method of producing such a crown wheel

ABSTRACT

In the production of a crown wheel which can mesh with a cylindrical pinion if the axes of rotation of the crown wheel and the pinion are not parallel, the workpiece from which the crown wheel is produced and a generating tool rotate at a ratio in the speed of rotation which corresponds to the proportion of the number of passes of the tool and the number of teeth of the crown wheel to be produced, and the tool is brought into engagement with the workpiece and is moved in such a way along the workpiece in a direction parallel to the axis of rotation of the cylindrical pinion that the tool works the tooth flanks of the crown wheel to be produced. When the center point of the tool is moved in a direction parallel to the axis of rotation of the cylindrical pinion which can mesh with the workpiece, and the teeth of which form an angle β with the axis of rotation of the cylindrical pinion, the workpiece acquires an additional rotation which is proportional to the product of this movement and the tangent of the tooth angle β of the cylindrical pinion. During this movement the angle between the axis of rotation of the tool and the plane through the axis of rotation of the cylindrical pinion parallel to the axis of rotation of the workpiece is constant.

The invention relates to the field of mechanical engineering in whichgear wheels which can mesh with a cylindrical pinion are produced, andin which the axes of the pinion and the gear wheel to be produced forman angle with each other. Such gear wheels are known under the name ofcrown wheels when the shaft angle is approximately 90°.

The method of producing such crown wheels accurately by, for example,milling is known.

However, until now it has only been possible to work crown wheels whichcan mesh with pinions provided with straight toothing, but there is aneed for crown wheels which can mesh with pinions provided with obliquetoothing, inter alia because this permits a greater torque to betransmitted between pinion and crown wheel. The shape of the teeth ofsuch crown wheels is complex, in view of the fact that the tooth isspiral and the angle which the tooth space forms with the radius vectortowards the centre of the crown wheel is not constant.

The invention relates to a method in which such a crown wheel can beworked by means of a generating tool, and to a tool by means of whichsaid method can be carried out.

The method according to the invention is a method of producing a crownwheel which can mesh with a cylindrical pinion if the axes of rotationof the crown wheel and the pinion are not parallel, in which theworkpiece from which the crown wheel is produced and a generating toolrotate at a ratio in the speed of rotation which corresponds to theproportion of the number of passes of the tool and the number of teethof the crown wheel to be produced, and in which the tool is brought intoengagement with the workpiece and is moved in such as way along theworkpiece in a direction parallel to the axis of rotation of thecylindrical pinion that the tool works the tooth flanks of the crownwheel to be produced. When the center point of the tool is moved in adirection parallel to the axis of rotation of the cylindrical pinionwhich can mesh with the workpiece, and the teeth of which form an angleβ (described more particularly below) with the axis of rotation of thecylindrical pinion, the workpiece acquires an additional rotation whichis proportional to the product of this movement and the tangent of thetooth angle β of the cylindrical pinion. During this movement, the anglebetween the axis of rotation of the tool and the plane through the axisof rotation of the cylindrical pinion parallel to the axis of rotationof the workpiece is constant.

The angle between the axis of the tool and the radius vector towards thecentre of the crown wheel remains constant while this method is beingcarried out.

It has been found possible to carry out the method according to theinvention on existing machines. In this case a tool whose pitch angle isequal to the tooth angle of the pinion can be used, but it has also beenfound possible to reduce this pitch angle. An advantage of reducing thepitch angle γ is that the number of passes of the tool is reduced, whichmeans that the number of teeth over which the workpiece rotates duringone revolution of the tool is reduced. This means that at a givenmaximum speed of rotation of the turntable on which the workpiece ismounted it is possible to increase the speed of the tool while theworking speed between tool and workpiece remains the same, with theresult that the working time of the workpiece is reduced.

The invention will be explained in greater detail below with referenceto the drawing.

FIG. 1 shows a cylindrical pinion whose tooth direction is the same asthe axis direction, so that this is a matter of straight toothing.

FIG. 2 shows a cylindrical pinion whose tooth direction forms an anglewith the axis direction, i.e. oblique toothing.

FIG. 3 shows a cylindrical pinion with oblique toothing having thereinthe transverse plane.

FIG. 4 shows a cylindrical pinion with oblique toothing having thereinthe normal plane.

FIG. 5 shows the known tool for working crown wheels which can mesh witha pinion with straight toothing.

FIG. 6 shows the tool according to the invention for working crownwheels which can mesh with pinions with oblique toothing.

FIG. 7 shows a crown wheel being worked by means of a generating tool.

FIG. 8 shows the relative path of a point of the machining surface ofthe tool relative to the tooth flank of the crown wheel for the diameterof the tool at which the pitch angle γ is equal to the tooth angle β ofthe pinion meshing with the crown wheel.

FIG. 9 shows the relative path of a point of the machining surface ofthe tool relative to the tooth flank of the crown wheel for the diameterof the tool at which the pitch angle γ is not equal to the tooth angle βof the pinion meshing with the crown wheel.

FIG. 10 shows the machining profile in cross-section X--X of FIG. 6,with the corrections made in the profile.

FIG. 11 shows in top view a crown wheel which can mesh with astraight-toothed cylindrical pinion being worked with a generating toolaccording to FIG. 5.

FIG. 12 shows in top view a crown wheel which can mesh with anoblique-toothed cylindrical pinion being worked with a generating toolaccording to FIG. 6.

FIG. 13 shows in side view the working according to FIG. 11.

FIG. 14 shows in side view the working according to FIG. 12.

FIG. 15 shows in top view the shape of the tooth space of a crown wheelwhen the pinion and crown wheel axes intersect each other.

FIG. 16 shows in top view the shape of the tooth space of a crown wheelwhen the pinion and crown wheel axes cross each other.

FIG. 17 shows in top view a crown wheel which can mesh with anoblique-toothed pinion being worked by a generating tool whose pitchangle is smaller than the tooth angle of the oblique-toothed pinion.

FIG. 18 gives a diagrammatic top view of how a crown wheel which canmesh with an oblique-toothed pinion is worked by a generating tool whosepitch angle is smaller than the tooth angle of the oblique-toothedpinion.

FIG. 19 shows the machining profile in section X--X of FIG. 6, with thecorrections made in the profile, and in which the pitch angle of thetool is smaller than the tooth angle of the oblique-toothed gear wheel.

FIG. 20 shows diagrammatically the calculation of the corrections of themachining profile of the tool.

The corresponding parts are shown by the same reference numbers in thevarious figures.

A cylindrical pinion with straight involute toothing, as shown in FIG.1, has an axis 1 and a pitch circle 2, the teeth being indicated by 3and the tooth spaces by 4. The shaft hole 5 is used for clamping thepinion during working or during use of the pinion.

FIG. 2 shows a cylindrical pinion with the same number of teeth and thesame tooth shape as in the pinion of FIG. 1, in which the teeth aredisposed helically. The pinion of FIG. 2 is thus an oblique-toothedpinion.

FIG. 3 shows the transverse plane 6, which is the plane at right anglesto the axis 1.

FIG. 4 shows the normal plane 7, which is at right angles to the toothflank. The production of an oblique-toothed pinion such as that shown inFIGS. 2 to 4 is based on a specific number of teeth, a tooth angle β, aspecific profile shape and a normal module m, i.e. the module in thenormal plane 7. As is known from gear theory, the normal module foroblique toothing is established in the shape of the tool by means ofwhich an oblique-toothed gear wheel is produced, the normal moduleremaining the same if the tooth angle β and the number of teeth vary.

A known generating tool by means of which crown wheels meshing withstraight-toothed pinions can be produced is shown in FIG. 5. In thiscase the axis of rotation of the tool is indicated by 8, and the bore bymeans of which the tool is clamped in the working machine is indicatedby 9. The centre of the toothing of the pinion meshing with the crownwheel, from which the shape of the machining profile of the tool isderived, lies on a circle 10 around the axis of rotation 8. The pitchcircle of this toothing is indicated by 11.

The outside diameter of the tool is indicated by 26, while the pitchcircle diameter of the tool, i.e. the largest diameter on the toothingof the tool which corresponds to the pitch circle of the pinion, isindicated by 25. As can be seen in FIG. 5, this is a single-pass tool,in the case of which the workpiece from which a crown wheel is producedrotates over one tooth during a full revolution of the tool. The toolhas helical ribs 12 and grooves 13 on the outer periphery, with a pitchangle 14 relative to the plane in which the circle 10 lies around theaxis of rotation 8.

FIG. 6 shows a generating tool according to the invention which hasdimensions which are comparable to those in FIG. 5. In this case thepitch angle 14 of the ribs of the machining profile of the tool issubstantially greater than in the case of the tool in FIG. 5, with theresult that the ribs 12 have also acquired a totally different shape.This large pitch angle 14 is achieved by making the toothing, which isderived from the transverse section 6 (see FIG. 3) of theoblique-toothed pinion meshing with the crown wheel to be produced,rotate about the axis of rotation of the tool with the centre on thecircle 10, in which case the pinion rotates over a whole number of teethfor each rotation about the axis of rotation 8 of the tool. This numberof teeth over which the pinion rotates is selected in such a way thatthe pitch angle 14 corresponds as far as possible to the tooth angle βof the oblique-toothed pinion.

FIG. 7 shows the way in which the tool according to the invention isused for working a workpiece. In this case reference number 19 indicatesthe workpiece to be worked, and arrow 18 indicates the rotation of saidworkpiece about axis of rotation 23 during the working. The tool 16,which rotates in the direction of arrow 17 about axis of rotation 15,has a machining surface 21 consisting of ribs 22 which lie helicallyaround the outer periphery. The ribs 22 work the tooth spaces 24, andthe tool has a feed direction 20 towards the axis of the pinion shaftmeshing with the crown wheel to be produced. The speeds of rotation ofthe tool and the workpiece are such that when there is a standstill infeed direction of the tool, on one full revolution thereof the workpiecerotates the same number of teeth as the number of passes n of the tool.

The tool moves in the direction of the axis of rotation of the pinionmeshing with the crown wheel. With this feed movement the workpiece isgiven an additional rotation, which adapts the position of the workpieceto the rotation of the transverse section of the pinion occurring as aresult of the tooth angle on movement in the direction of the axis. Thisadditional rotation is proportional to the product of the tangent of thetooth angle β and the feed `a` in the direction of the axis of rotationof the pinion.

FIG. 8 indicates by line 27 the tooth flank to be worked, while 28indicates the relative path of one point on a specific diameter of themachining surface in one cross-section of the crown wheel tooth when thetool is moving through the tooth space. In the example shown here, thepitch angle of the tool is equal to the tooth angle of the pinionmeshing with the crown wheel. As can be seen from FIG. 8, the relativemovement in this case is a straight line which in its lowest point 29ends on the expected tooth flank.

FIG. 9 shows the situation in the case of the other diameters, therelative path being indicated by 28. The machining surface runs to thetooth flank to be produced at 30 and leaves the area at 31. In this casethe path is elliptical, with the lowest point 29 on the tooth flank 27to be produced. As can be seen in FIG. 9, a part of the flank to beproduced is cut away in this case. In FIG. 9 the deviation of the toothflank is indicated by 32. This deviation can be calculated, so that themachining surface can be adapted in such a way that the deviation 32 isminimized or disappears.

The adaptations to the machining surface are admissible, because eachpoint of the crown wheel tooth flank is made by one point of themachining surface of the tool. It has been found here that even when thetooth flank is worked during the feed or the run-out, each point of thetooth flank is worked only by one point of the machining surface. It hasalso been found that the adaptations to the machining surface depend onthe pressure angle of the crown wheel and are more or less independentof the number of teeth of the crown wheel, so that all crown wheelswhich can mesh with a particular gear wheel can be made with one tool.

FIG. 10 shows the section X--X of FIG. 6, indicating the correctionspossibly occurring in the machining surface. The centre of the machiningsurface is indicated by line 33, and the centre of the toothing of thissurface is indicated by 34. This point 34 lies on the circle 10, whichis situated at right angles to the axis of rotation of the tool and isshown in FIG. 6. The side of the tool is indicated by 35, and the outerperiphery of the tool by 36. The profile 37 indicated by the broken lineis the uncorrected profile of the machining surface, as it appears atright angles to the surface. This profile 37 corresponds to the normalprofile of the oblique-toothed pinion meshing with the crown wheel to beworked.

The line 38 indicates the diameter of the machining surface and, inaddition to the rotation of the workpiece, as a result of the axialmovement of the tool in the direction of the axis of rotation of thepinion meshing with the crown wheel, there is an equal rotation in theopposite direction which is produced by holding the point of the toolsituated at that diameter in one tooth cross-section. In the points 39of the machining surface the relative path of the tool, in relation tothe workpiece corresponds to the situation shown in FIG. 8, and is thusa straight line, with the tooth flank to be produced as the lowestpoint. In the points of the profile 37 away from point 39 the relativepath of the tool in relation to the workpiece corresponds to thesituation shown in FIG. 9, the lowest point of the elliptical path lyingon the tooth flange and, a part of the tooth flank being cut away beforeor after the lowest point is reached. The tooth flank is thus workedduring the feed or the run-out of the tool.

By calculating the deviation occurring in the case of this workingduring the feed or run-out, the adaptation to the machining surfacewhich is necessary in order to allow a correct crown wheel still to beproduced can be calculated. Line 40 indicates what this profile lookslike. It can be seen from this that the corrections to be applied arelimited through the fact that the tooth head 41 of the machining profilenarrows. The size of the possible corrections is limited through thefact that said tooth head 41 must be a minimum thickness and may not beovercut.

FIGS. 11 and 13 show a crown wheel which can mesh with astraight-toothed pinion being worked by the known tool shown in FIG. 5.The crown wheel 42 rotates about its axis of rotation 43 in thedirection 44. The crown wheel is provided with straight toothing 45which can mesh with a straight-toothed cylindrical pinion. The workingis carried out by tool 46, which rotates about its axis of rotation 47in the direction 55, and which is moved from the outside to the insidediameter in the direction 54, the centre point of the miller moving inplane 48. The rotations of tool and workpiece are coupled together inthe proportion of the number of passes of the tool and the number ofteeth of the workpiece. The flanks of the crown wheel are worked by thetool in the region 49, namely where the miller is deepest in engagementwith the workpiece.

FIGS. 12 and 14 show a crown wheel being worked by a tool of the typeshown in FIG. 6, in the case of which a generating process is applied towork a crown wheel which can mesh with an oblique-toothed cylindricalpinion. In this case the crown wheel 42 is provided with spiral toothing50. The tool 51 is provided with a machining surface and moves from theoutside diameter to the inside diameter in the direction 56. The crownwheel rotates about axis of rotation 43 in the direction 44, and thetool rotates about axis of rotation 52 in the direction 57. Therotations of tool and workpiece are coupled together in the proportionof the number of passes of the tool and the number of teeth of theworkpiece. The tool 51 is provided with a machining profile, in whichthe pitch direction coincides with the direction of the tooth angle ofthe pinion meshing with the crown wheel. Due to the fact that the flanksof the crown wheel toothing are also worked during the feed and run-out,the working does not occur only in the lowest point of the tool path,but over the larger region 58.

During the working, the distance 53 between the axes of rotation 52 and43 of tool and workpiece changes, with the result that the rotation ofcrown wheel relative to tool is influenced: for the spiral shape of theteeth means that when the tool is at a standstill the crown wheel mustrotate when the distance 53 changes.

FIG. 15 shows the spiral shape of the teeth of the crown wheel. In thiscase 59 is the axis of the pinion meshing with the crown wheel 42, withan outside diameter 60. The centre of the crown wheel tooth is indicatedby 61. During movement along the centre of the crown wheel tooth fromthe point on the outside diameter 60 to the point 62, the rotation ofthe workpiece about its axis is proportional to the product of themovement `a` in the direction of the axis of rotation 59 of the pinionmeshing with the crown wheel and the tangent of the tooth angle β ofsaid pinion.

The examples described above always show the embodiment in which theaxis of rotation of the pinion meshing with the crown wheel intersectsthe axis of rotation of the crown wheel. However, as shown in FIG. 16,it is also possible for these axes to cross each other, in which casethe pinion is placed off-centre at a distance b. The axis of rotation 59in this case crosses the axis of rotation 43 of the crown wheel 42 at adistance b. For the rest, the same indications apply in FIG. 16 as thosein FIG. 15, the main difference being that the spiral shape of thecentre of the crown wheel tooth 61 is directed more radially.

Since the tooth flanks are partly worked during the feed and run-out, itis necessary for the centre of the tool to be taken to the desired toothdepth over a larger area than the width of the toothing. This isindicated by distance 83 in FIGS. 15 and 16.

The tool according to the invention can be either a miller or a grindingworm. The machining faces of the miller are preferably placedapproximately at right angles to the direction of the ribs of the tool,so that the clearance faces lie approximately at right angles to themachining face. This produces the maximum strength and stability of thecutting edges.

FIGS. 17 and 18 show a further development of the idea of the invention.In this case crown wheel 64, which can mesh with an oblique-toothedcylindrical pinion 66 with tooth angle β, is worked by tool 65, in whichthe direction of the profile of the machining surface forms an angle γdeviating from the tooth angle β with the plane 69 at right angles tothe axis of rotation of the tool. In this case workpiece 64 and tool 65are positioned relative to each other in such a way that the directionof the machining surface corresponds approximately to the direction ofthe teeth of the pinion 66 in the plane of the toothing of the crownwheel.

FIG. 18 shows all this diagrammatically. The crown wheel 64 to be workedis provided with spiral toothing, which can mesh with the cylindricalpinion 66, which can rotate about the axis of rotation 67. This axis ofrotation 67 determines the crown wheel 64, and in the situation shown inFIG. 18 runs through the centre 68 of the crown wheel 64.

The toothing on the pinion 66 meshing with the crown wheel forms anangle β with the axis of rotation 67. The generating tool 65 is placedat a constant angle with the axis of rotation 67 of the cylindricalpinion, in such a way that the machining profile lies in approximatelythe same direction as the toothing of the crown wheel. This means thatthe machining profile forms approximately an angle β with the axis ofrotation 67. The plane through the centre of the machining profile andat right angles to the axis of rotation 82 of the tool is indicated by69. The direction of the ribs of the machining profile forms a pitchangle γ with plane 69, so that the angle 84 between the axis of rotation82 of the tool and the plane in which the axis of rotation 67 lies, andwhich is parallel to the axis of rotation 68 of the crown wheel, isapproximately (90°+γ-β).

It can be seen in FIGS. 15 and 16, and it can also be deducedtheoretically, that the spiral teeth have a changing angle with theplane through the axis of rotation 59 of the pinion, and parallel to theaxis of rotation 43 of the workpiece. However, it is found that theangle 84 between the axis of rotation of the tool 82 and the planethrough the axis of rotation 59 of the pinion, and parallel to the axisof rotation 43, must be kept constant. The angle between the radiusvector to the centre of the crown wheel and the centre of the tool isconsequently also constant.

The pitch angle γ is smaller than β in the example of an embodiment,with the result that the corrections which have to be made in theprofile of the tool are greater than in the case of the situationdescribed in FIGS. 12 and 14. The possibility for making the correctionsis limited, with the result that the possibilities for reducing thepitch angle γ of the tool are also limited.

FIG. 19 shows a cross-section which is comparable to the cross-sectionin FIG. 10. The corrections which can be made in order to achieve a goodworking as in the situation shown in FIGS. 17 and 18 are indicateddiagrammatically in the profile. In the case of such a working, thetooth angle β can be, for example, approximately 30°, in which case thepitch angle γ can then be approximately 12.5°. The dotted line 37indicates the original normal profile of the oblique-toothed pinion,while the corrections made in the profile are indicated by 70. At 71 thecorrection is so great that the head of the working tooth is overcut,and a part of the flank of the crown wheel teeth will not be worked. Nodiameter at which the relative movement between workpiece and tool is astraight line can be established, because said diameter lies outside theworking profile.

The size of the necessary corrections of the profile is calculated onthe basis of the permitted pressure angles on the crown wheel. If theseare limited, for example from 10° to 43°, the size of the corrections islimited. When the permissible corrections are being established, thesize of the fillet of the crown wheel to be worked is also important,for if it is made too small, there is the risk of a pinion becomingjammed in the crown wheel, which is undesirable.

The calculation of the corrections to be made can be carried out, interalia, by comparing the numerically established shape of the crown wheelteeth to be generated by means of the generating process with thenumerically established shape of the machining profile for each relativeposition which tool and workpiece can assume relative to each other.Successive steps are thus always taken both in the lengthwise directionof the tooth space and in the direction of rotation of the tool. Thiscalculation is shown diagrammatically in FIG. 20, the data 72 of thetool and the transmission being taken as the starting point. These dataare, for example, the data of the characterizing pinion, such as numberof teeth, tooth angle, profile correction and the like, and the millerdata such as diameter and pitch angle. The shape of the tooth flanks andof the root of the tooth of the crown wheel is calculated in block 73using these data. The shape of the teeth of the characterizing pinion iscalculated in block 74, and the shape of the machining profile of thetool in a particular cross-section in block 75.

Using the tool data and the workpiece data, the relative movement of themachining profile of the tool and the tooth flank and the fillet of theworkpiece is calculated in 76. In 77 they are compared with each other,and the corrections to be made in the machining surface are calculated.These corrections are compared in 78 with earlier calculatedcorrections, and the most representative are established. Thecalculation is then repeated after the tool has been rotated slightly in81, and this repetition continues until the tool has been positioned inall relevant positions relative to the tooth space.

After all corrections which have been found necessary have beenincorporated, it is checked in 79 by means of a simulation, which isalso carried out by calculation, whether the tooth flanks are beinggenerated correctly. If adjustments are necessary, the calculation iscarried out again from 72. If no further adjustments to the calculatedcorrections are necessary, the corrected flank is calculated, and anumerical data file is established in 80. The tool can be made with theaid of this numerical data file.

It has been found from the calculations carried out that in thesituation in which the pitch angle γ is equal to the tooth angle β thenecessary corrections are smaller then when the tool diameter isgreater. In the event of the pitch angle not being the same as the toothangle, the corrections are generally smaller in the case of a smallertool diameter. It is also found that the corrections depend on the feeddirection, so that different tools are necessary for working operationsin the same or in opposite directions.

This data file can be used in, for example, the making of a milling toolin which the milling cutters are placed around the periphery of thetool. The milling cutters are then made through the desired profile bymeans of spark erosion, the wire position being derived from thenumerical data file.

If the tool is a grinding tool, the tool can be formed in the mannerdescribed in the published international patent application WO 92/11,967in the name of Applicants which application is incorporated by referenceas if fully set forth herein.

I claim:
 1. Method for producing a crown wheel by means of a continuousgenerating process, which crown wheel can mesh with a helical toothedcylindrical pinion with a helix angle β in situations wherein a firstaxis of rotation of the crown wheel and a second axis of rotation of thepinion are non-parallel, comprising the steps of rotating a workpiecefrom which the crown wheel is produced about the first axis, rotating agenerating tool about a third axis, said generating tool having a numberof passes of cutting edges lying on the periphery of the tool as ridgesand grooves under a lead angle γ with a plane perpendicular to the thirdaxis, the speed of said rotation in a fixed ratio with respect to therotation speed of the workpiece and the number of passes of cuttingedges of the generating tool, bringing the tool into engagement with anactive tooth flank of the workpiece which is limited by an inside radiusand an outside radius, and moving the tool along the work piece in adirection parallel to the second axis, wherein during movement of thetool in the direction parallel to the second axis the workpiece isadditionally rotated over a rotation angle which is proportional to theproduct of the distance of movement in the direction parallel to thesecond axis and the tangent of the helix angle β of the cylindricalpinion, and that during this movement the angle between the third axisand a plane through the second axis and parallel to the first axisremains constant.
 2. Method in accordance to claim 1, wherein duringmachining of the flanks of the workpiece, the tool follows a path whichis longer than the difference between the outside and inside radius ofthe workpiece.
 3. Method in accordance with claim 1 wherein the anglebetween the third axis and the plane through the second axis andparallel to the first axis is essentially equal to 90°+γ-β, wherein γ isthe lead angle of the generating tool and β is the helix of thecylindrical pinion.
 4. Tool for producing crown wheels which can meshwith a helical pinion having a helix angle β, by means of a continuousgenerating process, wherein a generating tool and a workpiece rotatewith a constant ratio of the speeds of rotation and move in such a wayrelative to each other that the tool machines the workpiececontinuously, said tool comprising a disc which is rotatable about itsaxis, and being provided with machining elements on the peripherythereof; the cutting edges of the machining elements lying in the outersurface of a profile determining the shape of the teeth of a crown wheelto be produced with the tool, said profile extending essentiallyhelically over the periphery of the disc with a lead angle γ, eachcross-section of the helical profile having a center point which lies ona circle in a plane perpendicular to the axis of rotation of the tool,the center point of the circle lying on the axis of rotation, theprofile of the cutting edges of said tool corresponding to the normalprofile of said helical pinion.
 5. Tool according to claim 4, whereinthe profile is determined by the lead angle γ, the helix angle β and thefeed direction of the tool.
 6. A method for producing a crown wheelwhich can mesh with a helical pinion having a helix angle β, by means ofa continuous generating process, said method comprising the steps of:(a)rotating a workpiece at a first speed of rotation about a first axis,said workpiece having an inside radius and an outside radius, thedifference between said inside radius and said outside radius defining atooth depth; (b) rotating a generating tool at a second speed ofrotation about a second axis of rotation, said first and second speedsof rotation having a constant ratio, said generating tool comprising adisc rotatable about the second axis and comprising a periphery with aprofile corresponding to the helical pinion and having a lead angle γ,each cross-section of said profile having a center point which lies on acircle in a plane perpendicular to the second axis of rotation, thecenter point of the circle lying on the second axis of rotation; and (c)applying the tool to the workpiece and moving the tool through saidtooth depth to produce the crown wheel.